Question

1. For each of the situations described below: A. Draw a picture of the situation and circle the object under consideration. B. Draw a free-body diagram of the object. C. Indicate for each force whether the work done by each force identified in the free-body diagram is positive negative or zero. a. A student rides an elevator up to her dorm room. Assume the elevator moves at constant speed. b. A mover pushes a crate at constant speed along the floor. c. A child slides at constant speed down a slide at the playground. d. A baseball player hits a ball pitched toward him. Consider the time when the bat is in contact with the ball.

Answer 1

a. The work done by the tension force is positive, while the work done by gravity is zero in the elevator scenario.

b. The work done by the applied force is positive, and the work done by friction is negative in the crate-pushing scenario.

c. The work done by friction is negative, while the work done by gravity is zero in the sliding-down-a-slide scenario.

d. The work done by the baseball player is positive, and the work done by air resistance is negligible in the baseball hitting scenario.

a. In the elevator scenario, where a student rides up at constant speed, the tension force pulling her upward does positive work since it is in the direction of motion. Gravity, acting downward, does no work because the motion is vertical, and the displacement is horizontal.

b. When a mover pushes a crate at constant speed along the floor, the applied force does positive work since it's in the direction of motion. Friction, opposing the motion, does negative work.

c. In the scenario of a child sliding down a slide at constant speed, the work done by friction, which opposes the motion, is negative. Gravity, the force responsible for the motion, does no work as the displacement is perpendicular to it.

d. In the baseball hitting scenario, when the bat is in contact with the ball, the work done by the baseball player is positive, contributing to the ball's kinetic energy. Air resistance is usually negligible for such short time intervals, so its work is often considered minimal.